1. Field of the Invention
The invention relates generally to electromagnetic tools for well logging. More particularly, the invention relates to improved antennas and electromagnetic tools having improved antennas.
2. Background Art
The measurement of dielectric constant (or electric permittivity) of formations surrounding a borehole is known to provide very useful information about the formations. The dielectric constant of the different materials of earth formations vary widely (for example, 2.2 for oil, 7.5 limestone, and 80 for water), so measurement of dielectric properties is a useful means of formation evaluation. For example, if the lithology and the water saturation of a particular formation are known, then the porosity may be determined if the dielectric constant of the formation could be obtained. Similarly, if the lithology and porosity are known, information as to the degree of water saturation can be obtained by measuring the dielectric constant of the formation.
A logging device that improved the art of measuring formation dielectric constant was the electromagnetic propagation tool as disclosed, for example, in the U.S. Pat. No. 3,944,910 (“the '910 patent”) issued to Rau and assigned to the present assignee. This patent discloses a logging device including a transmitter and spaced apart receivers mounted in a pad that is urged against the borehole wall. Microwave electromagnetic energy is transmitted into the formations, and energy which has propagated through the formations is received at the receiving antennas. The phase shift and attenuation of the energy propagating in the formations are determined from the received signals. The dielectric constant and, if desired, the conductivity of the formations can then be derived from the phase shift and attenuation measurements.
The configuration of antennas is an important aspect of successful operation of electromagnetic propagation logging tools. At a relatively high frequency of operation (for example 1100 MHz.), the signal attenuates quite rapidly. Therefore, it is important to have transmitting antennas that can efficiently generate energy and inject it into the formations, and to have receiving antennas that can efficiently receive energy that has propagated through the formations. Because the accuracy of the dielectric constant and conductivity measurements depends upon accurate measurements of attenuation and phase shift of the received signals, it is essential that the antennas operate in a stable manner over time and that the antennas are in, and remain in, a substantially balanced condition.
In the '910 patent, the antennas in the electromagnetic propagation logging device are cavity-backed slot antennas, which are filled with a dielectric material and include a probe that is an extension of the center conductor of a coaxial cable. The center conductor of the coaxial cable extends across the cavity-backed slot connects to the wall on the opposite side of the cavity-backed slot (see FIG. 2).
The probe (or conductor) of the cavity-backed antenna, as disclosed in the '910 patent, extends across the slot in a direction parallel to the longitudinal direction of the borehole. This configuration is known as a “broadside” array. U.S. Pat. No. 4,704,581 (“the '581 patent”), issued to Clark and assigned to the present assignee, discloses a similar logging device, but wherein the slot (cavity-backed) antennas have probes that extend in a direction perpendicular to the longitudinal direction of the borehole. This configuration is know as an “endfire” array. The endfire array exhibits a deeper depth of investigation and is less affected by tool standoff than the broadside array. On the other hand, the broadside array exhibits a stronger signal characteristic than the endfire array and may be preferred in relatively lossy (low resistivity) logging environment. Note that most electromagnetic logging tools have two or more receiver antennas, which facilitate the measurements of difference signals between the receiver antennas. Difference measurements cancel undesirable environmental (e.g., borehole) effects and simplify data analysis. However, one of ordinary skill in the art would appreciate that these measurements may also be performed with a single receiver antenna. In this case, the characteristics of the single antenna should be calibrated so that the true signals may be extracted from the raw measurements. With a single receiver antenna, it is more accurate to refer to the setup as a “mode” rather than an “array.” However, for simplicity, this description uses “array” to generally refer to a tool configuration that includes a transmitter and one or more receivers. One of ordinary skill in the art would appreciate that embodiments of the invention are applicable to tool configurations having one or more receivers.
An example of a logging device based on the teachings of the '910 and '581 patents is a electromagnetic propagation tool sold under the trade name of EPT™ by Schlumberger Technology Corp. (Houston, Tex.). A similar tool, called adaptable EPT™ (“ADEPT”), can provide either broadside operation or endfire operation during a given run, depending on the logging conditions. The ADEPT logging tool has two changeable pads, one containing a broadside antenna array and the other an endfire antenna array.
The EPT™ or ADEPT tools use cavity-backed antennas (or slotted antennas) arrays. Other related tools based on similar arrays include U.S. Pat. No. 4,698,572 (“the '572 patent”) issued to Clark. The '572 patent discloses electromagnetic logging tools incorporating slot antennas that have improved properties as compared with the conventional cavity-backed antennas. The slot antennas disclosed in this patent include tuning elements to improve the operation.
Furthermore, U.S. Pat. No. 5,434,507 (“the '507 patent”) issued to Beren et al. discloses electromagnetic logging tools with two-dimensional antenna arrays. The antenna arrays may comprise conventional cavity-backed antennas or cavity-backed antennas having two conductors arranged in a crossed-dipole configuration. The two-dimensional array of antennas makes it possible to image the formations surrounding the borehole. The above described patents, i.e., the '901 patent, the '572 patent, the '581 patent, and the '507 patents, are assigned to the present assignee and are incorporated by reference in their entireties.
Although the cavity-backed antennas have been very reliable in wire line applications, the rough working environment of logging-while-drilling (LWD) applications may need a new design of antennas. If the antennas are placed on a pad, which can rotate at a rate up to 120 RPM in a drilling operation, the antennas would have much higher wear rates as compared with the wire line environment. As a result, the antennas may have to be replaced often. Accordingly, it is desirable to have new design of antennas that can tolerate the harsh environments of an LWD operation and/or can be more easily replaced.